The Negative and Positive of Rake Angles in Files

It just seems to make sense to me to run a bur forward instead of backward.

In 1979, I read an article in 'Scientific American Magazine' entitled 'The Mechanisms of Abrasive Machining' and it got me started understanding why some files cut so much better than others.

The subject matter of the article was not about dentistry or endodontics but it explained the concept of rake angles in a way that ultimately helped me design the Fine-Cut files.

Up to this point it never occurred to me that endo instruments had much in common with tools used in machining, carpentry and stone-cutting.

The formal definition of a rake angle is "the angle between the leading edge of a cutting tool and a perpendicular to the surface being cut".

A pretty dry definition until you begin to dig into it.

Rake angles come in two varieties,

Positive and

negative.

If the leading edge of the blade is ahead of the perpendicular, the angle is, by definition, negative.

Examples of negative rake instruments are:

Reamers

K-files

K-Flex files

Diamond burs

NT-files, and

Burnishing burs

If the leading edge of the blade is behind the perpendicular, the angle is by definition, positive.

Examples of positive rake instruments are

my Fine-Cut files,

Hedstrom files,

Dynatrak files and

Most dental burs.

There are several characteristics of cutting that are important to us in dentistry and particularly in endo.

When a negative rake angle is used, you cut by applying downward pressure which creates a compression wave ahead of the blade.

It's sort of like spreading butter on your toast or scraping paint with a plane blade.

It requires a lot of pressure to keep the blade in contact with the surface being cut, that is, to keep it from riding up on the compression wave.

This is usually accomplished by pushing downward on this conical file into a conical preparation and using the Morse Taper Effect to create the lateral pressure, much the same way a log splitter works, the harder you push down, the more pressure is exerted laterally.

When the compression wave reaches its elastic limit it breaks away from the surface.

As you can see, this takes a lot of effort and a lot of pressure depending on the hardness of the surface.

A good example of this type of cutting would be running a carbide bur backwards.

This pressure and burnishing creates what is known as a smear layer, which consists of a layer of crushed dentin, necrotic debris and bacteria that gets burnished into the dentinal tubules and lateral canals.

When a positive rake angle is used, on the other hand, you cut by separating one molecule of material from the work piece and creating a chip that curls away from the edge of the blade.

This cuts so readily that you have to make an effort to keep the blade from digging into the work-piece.

It is a very efficient way of cutting and in endo produces little or no smear layer.

Dr. Jerry Harrison of Corpus Christi, Texas took the course in Houston and went back to Corpus to explain to the members of his study group what he had learned.

He took with him a butter knife, a pepper shaker and two sticks of butter.

An odd assortment of demonstration tools for a dental study group, or so it seemed.

He opened the first stick of butter, covered it with black pepper and then held the butter knife as though he were spreading butter on a slice of toast, that is with the leading edge of the blade ahead of the contact point with the butter, that is, with a negative rake angle.

He made several strokes and with each stroke he removed a little pepper, some more of the butter and burnished what remained of the pepper further into the butter.

In this way he gradually removed the pepper but in the process removed quite a bit of the butter, taking eight or ten strokes to remove all the pepper.

Then he took the second stick of butter and covered it with pepper but this time held the knife at a positive rake, much as you might to if you were scraping a piece of toast or sharpening a pencil with a pen knife, with the leading edge of the blade behind the contact point.

Being careful not to dig into the butter, he carefully scraped the pepper off the stick of butter.

It took him only one stroke and most importantly, neither removed much of the butter nor burnished pepper into the butter.

Many endo files have negative rake angles because they are essentially a round wire made triangular or square and then twisted to form a helical spiral.

When these instruments are applied to the inner surface of a root canal the metal-dentin interface forms a negative rake angle.

No matter whether you use Push-Quarter-Turn-Pull filing or Circumferential Filing or Balanced-Force filing, the effect is the same.

You cut by creating a compression wave of debris ahead of the blade that gets burnished into the dentinal tubules and lateral canals...and incidentally apical to the file tip.

This is the smeared layer you hear so much about in the dental literature.

Technique then requires that you remove more tooth structure to remove the smear layer and then you burnish some more.

Gradually you get the canal clean but it takes a lot of effort and removes much more tooth structure than is necessary.

There are a whole class of endo instruments that do not have these problems...

those that have a positive rake angle which includes of course, Fine-Cut Files.

They require very light lateral pressure, remove the surface layer with very little effort and pull out the debris created with each stroke and create very little if any smear layer.

It just makes a lot of sense to me to "run the bur forward instead of backwards".

These instruments lend themselves to circumferential filing and washed field technique, subject matter for several articles in the future.